Method for enhancing anti-reflective coatings used in photolithography of electronic devices

ABSTRACT

A method of fabricating an integrated circuit using photolithography and an antireflective coating. An antireflective coating is formed on a substrate wherein the antireflective coating is electrically polarizable. A photoresist coating is formed on the antireflective coating on a side opposite from the substrate and the photoresist is exposed to activating radiation. The antireflective coating is subjected to an applied electric field at substantially the same time as the photoresist is exposed to activating radiation. The radiation absorption coefficient of said antireflective coating is increased and the refractive index of said antireflective coating is changed to be substantially equal to the refractive index of said photoresist coating.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to semiconductor photolithography methodsand, more particularly, to antireflection coatings for use withphotolithography.

2. Description of the Related Art

One step in the fabrication of semiconductor devices such as integratedcircuits is the formation of a substrate pattern over a semiconductorwafer surface through photolithographic masking and etching. Aphotoresist coating over a substrate is selectively exposed toactivating radiation directed through a mask defining the desiredconductor pattern. After photoresist development, the photoresist layerconstitutes a relief image mask over the substrate. The relief imagemask defines open areas over the substrate in a desired image pattern tobe transferred to the substrate. The image is transferred to the surfaceof the substrate by surface modification of the substrate in a negativeimage of the pattern within the photoresist coating, such as by removalof a portion of the substrate by an etching process or by implantationof an atomic species into the substrate. The etching is often done in aplasma etch reactor in which a plasma of ions reacts with and etchesaway the exposed substrate. During these processes, the coating of thephotoresist in the image pattern functions as a protective mask toprevent surface modification of the substrate underlying the photoresistmask. The resolution of the image transferred to the substrate isdependent upon the resolution within the imaged photoresist coating.

There are factors in addition to the resolution capability of thephotoresist used that influence the quality or resolution of the imagetransferred to a photoresist masked substrate. For example, withreflective integrated circuit substrates, such as aluminum, exposure ofa photoresist coating causes reflection of diffused activating radiation(light) from the integrated circuit substrate back into the photoresistcoating. Standard photoresists are susceptible to surface reflectionswhich degrade the fine-line images required for integrated circuitmanufacture. This degradation occurs due to reflection of diffused lightfrom the integrated circuit substrate back into the photoresist layerresulting in exposure of the photoresist layer in areas where imaging isnot desired. Another common result of surface reflections is theformation of “notches” in conductive lines in certain regions because ofunwanted exposure of photoresist by reflected light. These “notches” cancause the device to fail, or even worse, to be unreliable.

To prevent reflection of activating radiation into a photoresistcoating, it is well known to provide antireflective layers (ARC's)between a substrate and a photoresist layer. These antireflective layerstypically comprise an absorbing dye dispersed in a polymer binder thoughsome polymers contain sufficient chromophores whereby a dye is notrequired. When used, the dye is selected to absorb and attenuateradiation at the wavelength used to expose the photoresist layer thusreducing the incidence of radiation reflected back into the photoresistlayer. During the conventional processing of an integrated circuitsubstrate coated with the combination of an antireflective layer and aphotoresist layer, the photoresist is exposed to activating radiationand developed to form a relief image, i.e., portions of the photoresistlayer are removed by development with a liquid developer and portionsremain as a mask defining a desired pattern. To alter the underlyingsubstrate, the antireflective layer must be removed to bare thesubstrate in a desired image. Removal of the antireflective layer may beby dissolution with a liquid that simultaneously dissolves both thephotoresist and the antireflective layer or by dry etching such as withan oxygen plasma.

Unfortunately, present antireflective coatings are less than 100 percenteffective and are often difficult to remove. Furthermore, removal of theantireflective coating often results in degradation of important deviceproperties and inconsistent performance of antireflection coatings dueto thickness variation and other factors limits performance ofphotolithography. Therefore, it is desirable to provide anantireflective coating and a photolithographic process that results inup to 100 percent efficiency and that can be more easily removed fromthe underlying substrate.

SUMMARY OF THE INVENTION

The present invention provides a method of fabricating an integratedcircuit using photolithography and an antireflective coating. Accordingto a preferred embodiment, an antireflective coating is formed on asubstrate wherein the antireflective coating is electricallypolarizable. A photoresist coating is formed on the antireflectivecoating on a side opposite from the substrate and the photoresist isexposed to activating radiation. The antireflective coating is subjectedto an applied electric field at substantially the same time as thephotoresist is exposed to activating radiation. The radiation absorptioncoefficient of said antireflective coating is increased and therefractive index of said antireflective coating is changed to besubstantially equal to the refractive index of said photoresist coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objects and advantages thereof, will bestbe understood by reference to the following detailed description of anillustrative embodiment when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 shows a flow chart depicting successive steps in thephotolithographic masking and etching of a layer such as a metal layeroverlying a semiconductor substrate.

FIGS. 2-8 show cross sections of a wafer at various stages in thefabrication process.

FIG. 9 shows a main chain polymer liquid crystal.

FIGS. 10A-10B show side chain polymer liquid crystals.

FIG. 11A depicts one type of main chain polymer liquid crystal.

FIG. 11B shows PHNA poly(hydroxynapthoic acid), a molecule suitable as amonomer.

FIG. 12A depicts a second type of main chain polymer liquid crystal.

FIG. 12B shows an example of a suitable mesogen, in this case PETpoly(p-phenyleneterephthalate).

FIG. 13 depicts a side chain polymer liquid crystal.

FIG. 14 depicts an example of a side chain and mesogen.

FIG. 15 shows a twisted side chain polymer liquid crystal.

FIG. 16 shows an example of an aromatic ring suitable to form mesogensor monomers for polymer liquid crystals.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a flow chart depicting successivesteps in the photolithographic masking and etching of a layer such as ametal layer overlying a semiconductor substrate. Referring to FIG. 2,the method is intended to create a conductor pattern in a conductivefilm 202 which overlies a semiconductor substrate 201 and is typicallyinsulated from substrate 201 by a dielectric layer 213 of a materialsuch as silicon dioxide. Referring again to FIG. 1, the first step (step110) of the method is to spin an antireflection coating (ARC) 203 on theupper surface of the conductive film 202; that is, to deposit a fluidantireflection coating on the surface of conductive film 202 and thendistribute it evenly over the surface of conductive film 202 by spinningsubstrate 201 in a manner well known in the art. Referring to FIG. 2,the thickness of antireflective coating 203 is preferably betweenapproximately 0.07 microns and approximately 0.15 microns with a mostpreferable thickness of approximately 0.1 microns. Antireflectivecoating 203 is comprised of a polymer liquid crystal material that iselectrically polarizable and may also contain a solvent such ascyclohexadone to facilitate adhesion and spreading on the wafer. Moredetail about this material is provided below.

As shown in FIG. 1, the second step (step 120) is to permit the fluidantireflection coating 203 to settle and harden. If a polymer precursoris used, the hardening is the stop of polymerization, and if the fluid203 is a polymer in a carrier, the hardening results from theevaporation of the carrier. Referring to FIG. 3, this step results in aplanarized upper surface 233 of coating 203.

Referring again to FIG. 1, the next step (step 130) is to coat the uppersurface 233 with a layer of photoresist 204. The photoresist isdeposited and distributed, again, by spinning, as is well known in theart, which results in a photoresist layer 204 shown in FIG. 4. Becauseof the planarization of the upper surface 233 of antireflection coating203, the photoresist layer 204 can be made to have a highly uniformthickness and a highly planar upper surface which are desired for sometypes of activating radiation.

The next step (step 140) of FIG. 1 is to selectively expose photoresist204 to activating radiation 206 while subjecting antireflective coating203 to an applied electric field 207 as shown in FIG. 5. A mask 205 withareas that are transmissive to the wavelength and type of activatingradiation 206 chosen is placed between photoresist 204 and the source ofactivating radiation 206 so that photoresist 204 will only be exposed toactivating radiation in desired areas. The directions of the appliedelectric field 207 should be approximately normal to the planes of theinterfaces between adjacent layers as shown in FIG. 5. It does notmatter whether the electric field 207 is up (as shown in FIG. 5) or downas long as the applied electric field 207 is approximately normal to theplanes of the interfaces between adjacent layers. The magnitude ofapplied electric field 207 is optimally between approximately 100 voltsand 200 volts DC. The properties of antireflective coating 203 will bechanged by this process such that the refractive index (n) ofantireflective coating 203 will become approximately equal to therefractive index of photoresist 204 and the extinction coefficient(k_(ARC)) will be in the range of approximately 0.2 to approximately0.5. In other words, n_(ARC)≅n_(resist) and k_(ARC)=˜0.2 to ˜0.5. Thusthe optical properties of antireflective coating 203 are tuned tophotoresist layer 204 above it.

Next, photoresist 204 is developed (step 150). As is known in the art,development of photoresist coating 204 produces openings 254 inphotoresist coating 204 describing the desired pattern to be formed inconducting layer 202 as shown in FIG. 6.

Next, antireflection coating 203 coating and conductive layer 202 areplasma etched (step 160 of FIG. 1). The type of plasma utilized dependson the type of material used as conductive layer 202. However, typicalplasma etches may be oxygen or fluorine based etch chemistries of a typewell known in the art. The etch stops on dielectric layer 213. Finally,after plasma etch (step 160 of FIG. 1), photoresist layer 204 and ARClayer 203 are removed by etching, as is well known in the art, so as toleave the patterned conductive layer 202.

Turning now to FIGS. 9-16, a more detailed description of the polymerliquid crystals suitable for use in ARC 203 is given. As mentionedabove, ARC 203 comprises an electrically polarizable polymer liquidcrystal. Polymer liquid crystals (“PLCs”) are a class of materials thatcombine the properties of polymers with those of liquid crystals. These“hybrids” show the same mesophases characteristic of ordinary liquidcrystals, yet retain many of the useful and versatile properties ofpolymers.

In order for normally flexible polymers to display liquid crystalcharacteristics, rod-like or disk-like elements (called mesogens) mustbe incorporated into their chains. The placement of the mesogens plays alarge role in determining the type of PLC that is formed. Main-chainpolymer liquid crystals (“MC-PLCs”) are formed when the mesogens 910 arethemselves part of the main chain of a polymer 920. An example of thestructure of a MC-PLC is shown in FIG. 9. Conversely, side chain polymerliquid crystals (“SC-PLCs”) are formed when the mesogens 1020 areconnected as side chains to the polymer 1010 by a flexible “bridge”(called the spacer) 1030. Examples of SC-PLCs are shown in FIGS. 10A and10B.

MC-PLCs are formed when rigid elements are incorporated into thebackbone of normally flexible polymers. These stiff regions along thechain allow the polymer to orient in a manner similar to ordinary liquidcrystals, and thus display liquid crystal characteristics. There are twodistinct groups of MC-PLCs, differentiated by the manner in which thestiff regions are formed.

The first group of MC-PLCs is characterized by stiff, rod-like monomers.These monomers 1110 are typically made up of several aromatic rings 1120that provide the necessary size. FIG. 11A shows an example of this kindof MC-PLC. FIG. 11B shows PHNA poly(hydroxynapthoic acid), a moleculesuitable as monomer 1110.

The second and more prevalent group of MC-PLCs is different because itincorporates a mesogen directly into the chain. The mesogen acts justlike the stiff areas in the first group. Generally, the mesogenic unitsare made up of two or more aromatic rings that provide the necessaryrestriction on movement that allow the polymer to display liquid crystalproperties. The stiffness necessary for liquid crystallinity resultsfrom restrictions on rotation caused by steric hindrance and resonance.Another characteristic of the mesogen is its axial ratio. The axialratio is defined to be the length of the molecule divided by thediameter (x=L/d). Experimental results have concluded that thesemolecules must be at least three times as long as they are wide.Otherwise, the molecules are not rod-like enough to display thecharacteristics of liquid crystals.

This group is different from the first in that the mesogens areseparated or “decoupled” by a flexible bridge called a spacer.Decoupling of the mesogens provides for independent movement of themolecules, which facilitates proper alignment. FIG. 12A shows a diagramof this type of MC-PLC. FIG. 12B shows an example of a suitable mesogen1220, in this case PET poly(p-phenyleneterephthalate). Notice theflexible spacer 1210 (methylene groups) and the stiff mesogen 1220(aromatic ring and double bonds).

SC-PLCs have three major structural components: the backbone 1310, thespacer 1330, and the mesogen 1320. FIG. 13 shows an example of a SC-PLC.The backbone 1310 of a SC-PLC is the element that the side chains 1320and 1330 are attached to. The alignment of the mesogens 1320 causes theliquid crystal behavior. Usually, the mesogen 1320 is made up of a rigidcore of two or more aromatic rings joined together by a functionalgroup. FIG. 14 shows a diagram of a typical repeating unit (mesogen1320) in a side chain polymer liquid crystal. Notice the spacer 1330 ofmethylene units and the mesogen 1320 of aromatic rings.

Like their main chain counterparts, mesogens 1320 attached as sidegroups on the backbone 1310 of SC-PLCs are able to orient because thespacer 1330 allows for independent movement. Notice, in FIG. 15, thateven though the polymer backbone 1310 may be in a tangled conformation,orientation of the mesogens 1320 is still possible because of thedecoupling action of the spacer 1330.

Another example of an aromatic ring suitable to form mesogens 1320 and1220 and monomers 1120 is the photochromic liquid crystal polyacrylatedepicted in FIG. 16.

In an alternate embodiment, ARC layer 203 is formed using chemical vapordeposition rather than a spin on method. In another alternateembodiment, photoresist layer 204 is formed using chemical vapordeposition.

In an alternate embodiment, the electrically polarizable liquid polymeris replaced with a magnetically polarizable material. The applied DCelectric field in step 140 is replaced with a magnetic field or analternating electric field with the field direction pointing in adirection substantially normal to the plane of the interface between ARClayer 203 and photoresist layer 204.

Although the present invention has been described with reference to aconducting layer and has particular relevance to metals because of theirhighly reflective surfaces, the present teachings apply to patterningnon-conductive and non-metallic layers as well. Furthermore, the presentinvention is not limited to the specific examples of polymer liquidcrystals given. Any electrically polarizable polymer liquid crystal willsuffice. However, the examples given are the presently preferred polymerliquid crystals.

The methods of forming individual layers in the wafer are given merelyas examples. Other methods of forming layers other than chemical vapordeposition and spin-on techniques are applicable as well

The description of the preferred embodiment of the present invention hasbeen presented for purposes of illustration and description, but is notintended to be exhaustive or limited to the invention in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. The embodiment was chosen and described inorder to best explain the principles of the invention the practicalapplication to enable others of ordinary skill in the art to understandthe invention for various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. A method of fabricating an integrated circuitcomprising the steps of: forming an antireflective coating on asubstrate wherein said antireflective coating is electricallypolarizable; forming a photoresist coating on said antireflectivecoating on a side opposite from said substrate; and exposing saidphotoresist to activating radiation while said antireflective coating isbeing subjected to an applied electric field at substantially the sametime wherein the radiation absorption coefficient of said antireflectivecoating is increased and the refractive index of said antireflectivecoating is changed to be substantially equal to the refractive index ofsaid photoresist coating.
 2. The method of claim 1 further comprising astep of developing said photoresist.
 3. The method of claim 2 furthercomprising a step of etching said antireflective coating and saidsubstrate to remove material from a first area.
 4. The method of claim 3further comprising a step of removing said antireflective coating andsaid photoresist coating.
 5. The method of claim 1 wherein said appliedelectric field is greater than 100 volts DC.
 6. The method of claim 1wherein said applied electric field is less than 200 volts DC.
 7. Themethod of claim 1 wherein said forming an antireflective coating isperformed using a spin on technique.
 8. The method of claim 1 whereinsaid forming an antireflective coating is performed using chemical vapordeposition.
 9. The method of claim 1 wherein said forming saidphotoresist coating is performed using a spin on technique.
 10. Themethod of claim 1 wherein said forming said photoresist coating isperformed using chemical vapor deposition.
 11. The method of claim 1wherein said antireflective coating comprises a polymer liquid crystalmaterial.
 12. The method of claim 11 wherein said polymer liquid crystalmaterial comprises a photochromic liquid crystal polyacrylate.
 13. Themethod of claim 11 wherein said polymer liquid crystal materialcomprises a main chain polymer liquid crystal.
 14. The method of claim11 wherein said polymer liquid crystal material comprises a side chainpolymer liquid crystal.
 15. The method of claim 1 wherein said electricfield is applied in a direction substantially normal to the interfacebetween said antireflective coating and said photoresist coating. 16.The method of claim 1 wherein said antireflective coating is about 0.07microns to about 0.15 microns thick.
 17. The method of claim 1 whereinsaid antireflective coating is approximately 0.1 microns thick.
 18. Themethod of claim 1, wherein the activating radiation is composed ofelectrons.
 19. The method of claim 1, wherein the substrate is glass orquartz.
 20. A method of fabricating an integrated circuit comprising thesteps of: forming an antireflective coating on a substrate wherein saidantireflective coating is magnetically polarizable; forming aphotoresist coating on said antireflective coating on a side oppositefrom said substrate; and exposing said photoresist to activatingradiation and subjecting said antireflective coating to an appliedelectromagnetic field at substantially the same time whereby theradiation absorption coefficient of said antireflective coating isincreased and the refractive index of said antireflective coating ischanged to be substantially equal to the refractive index of saidphotoresist coating.
 21. The method of claim 20 wherein said appliedelectromagnetic field is an AC electric field.
 22. The method of claim20 wherein said applied electromagnetic field is a magnetic field. 23.The method of claim 20 wherein said electromagnetic field is applied ina direction substantially normal to the interface between saidantireflective coating and said photoresist coating.
 24. A method offabricating an integrated circuit comprising the steps of: forming anantireflective coating on a substrate wherein said antireflectivecoating is polarizable; forming a photoresist coating on saidantireflective coating on a side opposite from said substrate; andexposing said photoresist to activating radiation while saidantireflective coating is being subjected to an applied field atsubstantially the same time wherein the radiation absorption coefficientof said antireflective coating is increased and the refractive index ofsaid antireflective coating is changed to be substantially equal to therefractive index of said photoresist coating.
 25. The method of claim24, wherein the antireflective coating is electrically polarizable andwherein the applied field is an electric field.
 26. The method of claim24, wherein the antireflective coating is magnetically polarizable andwherein the applied field is a magnetic field.